add TextEncoder & TextDecoder libdefs
add TextEncoder & TextDecoder libdefs

Summary: Closes https://github.com/facebook/flow/pull/1766

Reviewed By: samwgoldman

Differential Revision: D3865712

Pulled By: mroch

fbshipit-source-id: 80d90f025e787549b659ef7dfa97b6dbfbfbd5b1

fletcherw/flow,claudiopro/flow,jgrund/flow,ForbesLindesay/flow,ylu1317/flow,samwgoldman/flow,mroch/flow,gabelevi/flow,AprilArcus/flow,ForbesLindesay/flow,nmote/flow,mroch/flow,claudiopro/flow,AgentME/flow,gabro/flow,jgrund/flow,mroch/flow,popham/flow,chenglou/flow,MichaelDeBoey/flow,nmote/flow,MichaelDeBoey/flow,nmote/flow,MichaelDeBoey/flow,popham/flow,jamesgpearce/flow,samwgoldman/flow,ForbesLindesay/flow,chenglou/flow,gabro/flow,AgentME/flow,samwgoldman/flow,facebook/flow,jamesgpearce/flow,TiddoLangerak/flow,mroch/flow,fletcherw/flow,samwgoldman/flow,ylu1317/flow,nmote/flow,popham/flow,claudiopro/flow,gabro/flow,AprilArcus/flow,jamesgpearce/flow,facebook/flow,JonathanUsername/flow,jgrund/flow,jgrund/flow,TiddoLangerak/flow,popham/flow,popham/flow,mroch/flow,ForbesLindesay/flow,jamesgpearce/flow,gabelevi/flow,fletcherw/flow,ForbesLindesay/flow,gabelevi/flow,JonathanUsername/flow,TiddoLangerak/flow,AprilArcus/flow,TiddoLangerak/flow,mroch/flow,popham/flow,claudiopro/flow,gabro/flow,fletcherw/flow,ylu1317/flow,AgentME/flow,JonathanUsername/flow,AprilArcus/flow,mroch/flow,jgrund/flow,jamesgpearce/flow,nmote/flow,fletcherw/flow,gabro/flow,facebook/flow,chenglou/flow,gabelevi/flow,JonathanUsername/flow,MichaelDeBoey/flow,TiddoLangerak/flow,MichaelDeBoey/flow,AgentME/flow,samwgoldman/flow,jamesgpearce/flow,popham/flow,AgentME/flow,facebook/flow,claudiopro/flow,ForbesLindesay/flow,nmote/flow,facebook/flow,samwgoldman/flow,JonathanUsername/flow,chenglou/flow,JonathanUsername/flow,gabelevi/flow,fletcherw/flow,gabro/flow,samwgoldman/flow,jamesgpearce/flow,gabelevi/flow,JonathanUsername/flow,gabelevi/flow,nmote/flow,ylu1317/flow,facebook/flow,ylu1317/flow,facebook/flow,AprilArcus/flow,chenglou/flow,AprilArcus/flow,chenglou/flow,jgrund/flow